I noticed something which only a person reading a whole load of Eddie Izzard interviews in one go would ever notice: journalists always talk about his body. Is he fat or thin? Sorry, "heavy" or "sculpted"? Is he wearing men's clothes or women's, are they nice or scruffy?

He comes in for the kind of pretend-neutral, old-fashioned, nostrils-flared appraisal that women get and men almost never do. It's almost as if being a transvestite has unleashed the forces of sexism against him. I met him at the offices of his production company, in London, where he's promoting the DVD release of Believe, a film about the story of his life. He looks great, by the way, but let's leave it at that.

People are rarely the same in person as they are on stage; and sure, Izzard is grumpier and less performative, but the quality that made him famous is innate and not, I don't think, something that he can turn on and off. He is magnetic.

He can pretty much say anything. He does an impression of a German person using English slang which is so funny I can't hear his words on the tape under my honking laugh. I spend ages trying to decipher what he said, thinking it must be the most uproarious thing. And I get there in the end: "It's like if a German says 'be lucky, mate', that's really funny."

So that's what we're dealing with, a conversational Midas. In an interview just after he came out as a transvestite, a student newspaper asked him: "Are you worried you'll just be pigeonholed as the TV on the TV?" He replied, "It would be a pretty small pigeonhole, wouldn't it? Just me and the pigeon, going 'go on! Get out of my hole!'."

The sheer ease of the humour always made it seem as if the transvestism was no big deal: that his charm was so ineffable, not only was he instantly accepted, but he made everything to do with the transgender world acceptable.

He doesn't see it like this at all. Whenever he describes any aspect of his career or personality – his doggedness, his wilderness years doing street theatre (and that is a proper wilderness; we're not talking bit parts in Holby City), running 43 marathons in 51 days, the switch to acting, the gigs in French – he comes back to this: "Well, come on, I'm a transvestite with a career." He talks about it as a glass ceiling.

"It never looked like that much of an impediment, though."

"You're saying it's a glass rocket? You weren't there at the beginning, when people would say 'is he going to turn up wearing a dress?', sometimes it was 'we'll only have him if he's wearing a dress'." Of course, coming out as a transvestite is like anything else – people who are really good at it make it look easy. It doesn't mean it was.

Because it was hard, and is the touchstone to which Izzard returns whenever he wants to describe anything else that is a particular challenge, it is not a bad way to carbon-date his life. He realised he was a transvestite when he was four. His mother died when he was six. He decided to be an actor when he was seven.

He went to Sheffield University at 18, where he did accounting and financial management, he tells me, quite proudly. "But only for a year," I challenge. "I dropped it like Bill Gates dropped it, because I knew how to do it." This is weird, because I have his DVD in my hand, in which there's an interview with his tutor, who clearly says Izzard got kicked out for not doing enough work. Still, he'd have to do so much more to dent his likability than twist the reality of who dumped whom, between him and his university.He stayed at Sheffield, anyway, to direct student drama and take shows to the Edinburgh festival: Believe has some beautiful stuff on him as a student, a lot of which is frankly sweet.

Nevertheless, there is something arrogant and hard in his eyes as a 19-year-old, which has totally gone even by the time he's 24.

Obviously I don't know why this is, whether it's because he decided when he was 23 to tell his father, and slowly, the rest of the world, he was a transvestite, and from that point on knew more about being judged than judging; or maybe it was the softening grind of his vocation, the plugging away, trying to forge a career from swordfighting in Covent Garden plaza and being a crap unicyclist.

He gets nicer and nicer through the documentary, until by the end, his niceness reduces you to tears (when he did his marathons last year, the niceness was almost too much to bear – not raising money for charity. Any celebrity can do that. It was the sight of someone so human, so vulnerable and distressed, doing something so physically incredible; there's no way, as a viewer, you can distance yourself. You can't say "oh well, he's a nutter, or Superman, or an attention seeker". He is authentic, and resists dismissal). I still can't work out why he did it.

"There's a certain scene in [Believe] that does explain it. It may not be the explanation that you want. But it explains it to me. It's to do with my mother, and something happened, the combination of her dying and the determination gene, it went together, it fused into this determination thing. I've tried to do things in quite hard routes, deliberately. It makes you stronger, it means that things won't fall away so easily. You have a rocket career, you can rocket down again."

This happens time and again: Izzard starts off talking about determination, relentlessness, and segues into a remark about loss. His line is, if you're determined enough, if you work hard enough and cover enough ground, your valuable things won't be ripped from you. The interpretation he gives is that his mother's death made him more determined, which would be, not necessarily a logical progression of grief, but at least a productive one. I think, conversely, that it made him cling on to an idea that is fundamentally wrong: if you work hard enough, you can prevent tragedy. And actually, he knows that, because the scene he's talking about in the documentary is one in which he says that he keeps thinking, if he does more, it will bring his mother back.

There's a tension in him which is almost tangible, between keeping up the determination line, and admitting that his energy is futile, since its aim is to avert a disaster that has already happened, 41 years ago. "I can't deal with the death of my mother," he says, quite suddenly, in a neutral tone. "I just can't deal with it, because it's just too painful. I don't go there."

That all came up, not because we were discussing his mother, but because we were discussing his emotional "compression", or "firewall": the director of the film had to work quite hard to get behind it. Indeed, Sarah Townsend had a harder job even than that sounds, because she used to go out with Izzard.

The amount one knows about this comedian is really confusing: on the one hand, he has been this courageous figure for transvestism, being open about it even when it looked like it might torch his career, going very girly on a recent Canadian tour, even as he tries to consolidate his acting in Hollywood (where they are not at all big on alternative lifestyles, at least not for front men). In tandem with that, he's been very frank about his sexuality, since he's straight and people expected him to be gay.

"I felt everyone thought I was gay, and I could see it being a logical thing, you wear the clothes, you're gay. But I'm straight. I don't know why I'm straight, but I am. I like to be truthful and I'd like to get it down on record."

Yet, on the subject of actual relationships, he is probably the cagiest interview you will ever read.

He just won't say who he's going out with, ever. "Well, OK, I had a relationship with Sarah. And she made this [film]. And that sort of … we just wanted to keep separate spaces for each other." (There's too much to ask, here, otherwise, naturally, I would have said, "what on Earth is that supposed to mean, 'separate spaces for each other'?"). Did they split up before they made it?

"Before it had finished."

"You split up in the middle?"

"It went on for some time. But we still get on very well. It wasn't like some thunderous split-up, we're just exes that get on very well. That wasn't … it wasn't a problem, because it wasn't one of those blowouts. We get on very well, but we're now not … we're exes…"

It still doesn't totally stack up – the pair got together while Izzard was still a student, or at least in Sheffield being a sort-of student. And they didn't split up till the middle of the documentary. So they could have been together for 20 years, or they could have started making the documentary three years into the relationship and split up shortly after that, which would explain all the great early footage. His reluctance to give a timescale is endearing, even while it is also annoying.

As a standup, Izzard has been absent for the past couple of years, acting in films and the series, Riches, which didn't get off the ground but should have. He's obviously driven, but I came away with an inexact impression of which direction he's driving in. The film acting has never failed, but it's moving pretty slowly – smallish parts, in films that so far haven't ignited. A reviewer said, why would anyone go off to be a so-so actor, when they were a great stand-up comedian?

"But at the beginning, I was a so-so comedian!" he says, not offended by the remark (he brought it up). He has plans to do another standup tour in French, which consolidates both the fact that he's done it before, and his fiercely pro-European political stance.

This is almost the least comprehensible thing about him: of all the causes he would throw his considerable weight behind (metaphorically I mean, about the weight), he chose Europe. It seems like such an eddy. "You mean, like a good idea that's quite difficult?" (No, I meant like a current moving in the opposite direction to the main current, often in a circular motion).

"Well, I thought it was important. Like, why should I say I'm a transvestite? I could have kept it secret and lied about it. Instead, I thought, 'no, this is true, I'm going to tell people'. And I got the space for myself, maybe someone else can get some space out of that. Just like I thought with Europe, if we can do this, then other continents can learn to work together in some shape or form and … you know, you've got to get to a place where two billion people aren't having to live on two dollars a day."

I love the aim of solving global poverty, of course, but my first thought is, I wouldn't start from here. I wouldn't start off by trying to foster co-operation across Europe. It's such an incredibly long way round. However, I am won over: he's not talking about a single currency, he's just talking about the nations of Europe working together. It might not be a sufficient criterion for world peace, but it's certainly a necessary one.

Bear in mind, Izzard takes the long way round to everywhere, and he gets there, and he stays there. His is the way to make stuff happen. Of course it helps if you are magnetic. Not just anyone could unite Europe. But to borrow from Joan of Arc, 100 Eddie Izzards could do it. 10 of him could do it, with God on their side.

The Europe thing is a bit unfair, since his involvement in politics spans a wider support for New Labour as a whole, which he once likened to that of a backbencher – he was just there to be helpful. He never criticised, and even now as he talks about the last government, he says, "We did a lot of good things in New Labour, and we made some mistakes. It is difficult to run a country, any country." I doubt even Peter Mandelson still refers to New Labour as "we". He has said he'll stand for parliament in 10 years. I asked if he'd consider accelerating that. "Well, in an emergency ..."

Is it too soon to call him a national treasure? Is it too nationalist?